1. Field of the Invention
The present invention relates to high-quality digital printing in which objects are intermixedly printed, and more particularly, in which a printing process exhibits characteristic, statistically predictable defects which degrade quality. A system-specific and defect-specific function is applied to the digital signal anamorphically (different in the scan and process directions) to change the color or other attributes as a function of distance from an edge to pre-compensate for a defect and thereby increase quality.
2. Description of the Related Art
Digital color printers form a digital image for each of several separations, such as cyan, magenta, yellow, and black. The digital image instructs the printing mechanism of the printer in the amount of each color ink to deposit and the method of deposition at each addressable point on the page.
A digitally imaged page can consist of graphical objects such as text, lines, fills, pictures, etc, all imaged in ways which can be isolated from each other, can abut one another at one or more points, can partially overlap one another, or can completely overlap one another. The resulting printed page or graphic image is therefore made up of a patchwork of shapes representing the graphic objects, some of which are xe2x80x9cclippedxe2x80x9d by objects imaged later in the succession.
In practice, every color printing system has characteristic defects which can cause subtle problems that detract from achieving the highest possible quality color printing. For example, ink jet printing must handle excessive ink coverage which can cause bleeding or spreading of colors and paper distortion. Xerographic printing contends with a different set of problems which can detract from print quality. Examples are xe2x80x9chaloingxe2x80x9d, in which toner in one separation interferes with toner transfer at the same location in another separation, xe2x80x9ctentingxe2x80x9d, which is toner deletion caused by high toner pile casting a mechanical or electrostatic xe2x80x9cshadowxe2x80x9d which prevents correct development of abutting toner, trail-edge deletion and starvation, which cause toner deletion at certain edges, or misregistration between two colors. Many of these characteristic problems in printing systems can be traced to undesirable interactions between abutting colors on the page.
Despite known problems, the digital image sent to the printer has in the past assumed a perfect printing mechanism, and provided an ideal image to print. While increasingly sophisticated controls have been added to printing mechanisms to reduce defects and come closer to the perfect printer expected by the digital image, electro-mechanical defects in any printing system are still common and are to be expected at both the low end where system cost restraints preclude use of expensive controls and the high-end where production speeds challenge existing control systems.
Recent work has begun to look at modifying a digital image in advance in order to pre-compensate for expected problems in a printer. The work may be divided into two groupings. A first grouping of prior art does xe2x80x9cobject-based compensationxe2x80x9d, which predicts and pre-compensates for printing problems unique to each object type (text, fill, image, etc.). A second grouping of art does xe2x80x9ctrapping compensationxe2x80x9d, which predicts and pre-compensates for only one printing problem: misregistration between two abutting colors.
The first (object-based) grouping deals with isolated objects only; it does not look at problems caused by interactions or adjacencies between colors or objects on the page. Pre-compensation for printing problems is based purely on individual objects being printed, such as text, fill, or picture, without reference to other adjacent objects. Different object types have different predictable printing problems. For example, large uniform color fills can contain visible mottle in what should be smooth color, because the random noise of the print mechanism causes tiny variations in the amount of color put on the page. Text can show rough or fuzzy edges. Images can show unnatural colors. In order to pre-compensate for each special problem, the type of image processing to be done is changed as each different object type is processed for printing. This object-specific rendering pre-compensates for expected predicted printing problems by using different processing for different image types.
For example, when a large color fill is being prepared for imaging, it is possible to switch to a xe2x80x9cquieterxe2x80x9d halftone that will not show mottle so much. Similarly, if text is being printed, processing which emphasizes sharp edges to overcome limitations in the printer""s resolution is chosen. Graphics such as charts and graphs should have color processing which emphasizes bright, saturated colors even in printers of limited gamut, while pictures should be rendered with halftones that have a greater number of available tone values and color correction that emphasizes natural colors.
U.S. Pat. No. 5,634,089 to Kulbida et. al., provides background information on a digital processing method that differentiates among, for example, sampled images and text, and uses tags to process them differently so that sharp edges may be preserved in text while a fuller set of tone values is preserved in pictures.
U.S. Pat. No. 5,704,021 to Smith et. al., describes another system which automatically detects certain object types and changes the image processing accordingly so as to produce a better printed image.
U.S. patent Ser. No. 09/012,651 to Rumph, et. al., describes a more complete system called Object Optimized Rendering which can include hardware-assist for speed and compression, object tags that can be specified by the user or generated automatically and carried throughout the software and hardware image processing system, and even includes object-based measurement feedback to dynamically adjust color correction on an object type basis. This system, in addition to looking at object types as a guide to optimized rendering, also looks at object parameters such as the size of a fill in determining the types of rendering to do.
The second (trapping) grouping of prior art is more limited in scope in that it attempts only to pre-compensate for a single printer defect caused by adjacent colors: misregistration. If a printer misregisters between separations, an thin unwanted white or color line occurs when certain adjoining colors don""t abut perfectly. This second group of inventions doesn""t care about individual object types or a large range of printer defects as the first (object-based) grouping does. Instead, this group of prior art simply looks at the edge between two color areas, attempting to predict when two abutting colors could cause a thin line problem if the printing system misregisters. The solution used is to generate a fixed-width, constant color fill (a xe2x80x9cframexe2x80x9d or xe2x80x9ctrapxe2x80x9d) whose color and position is calculated with various methods from the two abutting colors, and to superimpose that new digital signal with the original signal to produce prints that show the misregistration problem less.
For example, the method of Taniguchi, described in U.S. Pat. No. 4,931,861, finds the border between abutting or overlapping colors using logical operations, shrinks one of the color borders, and defines a xe2x80x9clinkagexe2x80x9d portion as a frame or trap to be superimposed at the border between the two colors.
The method of Yosefi, described in U.S. Pat. No. 5,113,249 uses a set of automated rules as the basis for deciding, for each pair of abutting or overlapping colors, whether or not to create a frame, and, if so, the constant (fill) color to use and the position of the created frame. Yosefi describes rules to follow after finding an edge and knowing the two colors. There are 24 rules based on the two colors. Once the frames are made, they are combined with the original data to print an image having reduced defects.
U.S. Pat. No. 4,583,116 to Hennig et. al describes a trapping process that evaluates the weighted separations on both sides of an edge in order to determine which separation most contributes to the contour. The separation determining the contour is left unchanged. A constant color fill is constructed in a trap zone by replacing the remaining separations in that color with the corresponding separations of the adjacent color.
U.S. Pat. No. 5,131,058 to Ting et. al. converts a raster to an edge-based xe2x80x9coutlinexe2x80x9d representation. Then the outlines are expanded or contracted and the resulting image is rerasterized and merged with the original bitmap.
U.S. Pat. No. 5,295,236 Bjorge, et. al. describes starting from a PDL file, rasterizing and analyzing the result for edges, generating traps (frames) between edges of appropriate colors, and merging those traps into the original file by converting the trap rasters to PDL commands which are inserted into the original PDL file.
U.S. Pat. No. 5,204,918 to Hirosawa assumes vector data as input, describing the contours (edges). Contours are selectively modified to create two supplemental contours, offset at a specified distance from the original contour. A new color is computed for the entire offset region. The maximum (or minimum) density of the two sides is used in the correction region, and the correction region is filled with that density and used as a frame or trap between the two edges.
U.S. Pat. No. 5,402,530 to Boenke et al. uses object terminology as in the first (object-based) group of inventions, but belongs in this grouping because it only uses each object""s color and edge information to efficiently determine abutting color objects needing a trap region.
Similarly, U.S. Pat. No. 5,666,543 to Gardand and U.S. Pat. No. 5,542,052 to Deutsch et. al. identify edges between two colors in vector form and construct trap zones as PDL commands.
U.S. Pat. No. 5,542,052 to Deutsch, et. al assigns a relative darkness to each separation, with black being the darkest separation, cyan and magenta being middle darkness, and yellow being lightest. Then, a trap vector is drawn in a color which is derived from the two colors abutting each side of the edge.
U.S. Pat. No. 5,313,570 to Dermer, et. al. takes either raster or PDL input, and creates an intermediate, vector-based form that describes all the boundaries between two colors in the entire page. Trapping operations are applied to those boundaries to assign them the correct color, extent, and position, and the resulting traps or frames are overlaid onto the original image. The trapping rules themselves are independent of the process of finding and tracking the edges.
U.S. Pat. No. 5,668,931 to Dermer describes trapping rules. The overall concept is to have a set of evaluation methods, and for each candidate trap, let each of the evaluation methods decide whether it is an optimum trap. Each method ranks all of the candidate traps, and then the traps are scored, using the weighted sum of the rankings. In this way some evaluation methods are more likely than others. Candidate traps colors are derived from the two abutting colors on each side of the current edge being considered. The resulting traps are overlaid on the original image.
U.S. Pat. No. 5,613,046 to Dermer describes a user interface allowing the display of an image, selection of any color pair, and modification of the trapping behavior in terms of position or color. It also allows display of the effect of any of the 16 possible misregistrations on the selected color pair with or without the current trapping applied, and to iterate through the possible modifications, applying several possible traps to decide which is best. Users may specify the shape of the trap stroke, its position relative to the boundary between two colors, and a single fill color to be used.
U.S. Pat. No. 5,440,652 to Ting describes a process to find an edge and follow it, building a secondary edge a predetermined distance from the primary edge. The secondary edge will be used as the other side of the trap region. The secondary edge distance and the color of the region between (the trap zone) is determined by reference to a set of rules for color pairs.
U.S. Pat. No. 5,386,483 to Shibazaki discusses creating additional regions (frames) on boundaries between regions in the original image. The image is segmented into regions, each of a constant color. Each such region is assigned a region number, and a lookup table is used to store the correspondence between region number, and colors, including both cmyk, and rgb. RGB is used by the operator supervising the process with a display and mouse. The data is then run-length encoded, using runs of color table indices. The algorithm is multi-pass. On the first pass, an eight-neighbor window is used to form a pair of xe2x80x9cframexe2x80x9d regions along each color boundary. On subsequent passes, a four-neighbor set is used to extend the frame region. Finally, a color is assigned to each new region thus formed.
U.S. Pat. No. 5,241,396 to Harrington restricts its frame to correcting misregistration problems for xe2x80x9crichxe2x80x9d black (composed of cyan, magenta, yellow, and black separations). In these cases, the cyan, magenta, and yellow colors can (if misregistered) show beyond the black border, producing colored, fuzzy edges. The technique described creates a frame of single-black color (black only ink) at the edge by processing the raster such that the black separation images are eroded and then ANDed with each of CMY. The original black is then used as the black separation, producing the rich black on the interior and a black-ink-only frame at the edge.
Similarly, U.S. Pat. No. 4,700,399 to Yoshida finds edges and uses a different UCR along the edges from elsewhere to allow rich black without getting color bleeding along the edges of black objects. Thus colors are kept away from edges of black text by eroding the CMY content at the edge to produce a frame with less or no non-black color.
By contrast, the current invention combines both object and color information to predict and correct a wider range of adjacency problems in a novel way. Unlike the group one (object-based) inventions above which use object information to predict individual object printing problems, the current invention uses object information to help predict and solve object-adjacency printing problems. Unlike group two (trapping) inventions above which only correct for misregistration, the current invention significantly extends the range of adjacency problems that can be detected and corrected. Detection of a larger number of adjacency problems is made possible by including not just color information in predicting adjacency problems but also object information such as object type, object size in the scan and process directions, rendering intent, and other relevant object parameters. Pre-compensation/correction of a larger number of adjacency printing problems (beyond simply misregistration) is made possible by using a novel approach different from the simple trapping solution of adding a uniform-width, constant-color frame between two adjacent colors. Instead, a function is applied to an object edge that can change both its color and rendering hint anamorphically (that is, differently in the process or scan directions) as a function of the distance from the edge.
Thus, the current invention has four elements that go beyond the prior art.
First, in attempting to predict printing problems, the current invention goes beyond the prior art in that it does not look simply at adjacent colors as in the prior art. Instead, it looks for problematic adjacent objects (fills, text, sweeps, images, etc.), with predicted problems caused not only by colliding colors but also colliding attributes such as rendering intent (e.g., two different halftones adjacent to each other) or size in each direction (e.g., some printing problems only occur with objects greater than a certain size in a certain direction).
Second, in attempting to correct a predicted printing problem, the invention goes beyond the prior art in that it is able to apply a different solution in the process direction (down the page) or the scan direction (across the page). This is important because many, if not most, printing problems are asymmetric. Even misregistration, the focus adjacency problem of the prior art, generally is more severe in one direction than the other because misalignment along the scan is caused by different mechanisms than misalignment down the page. The current invention is able to respond to that asymmetry by applying its solutions anamorphically (differently in the scan and process directions).
Third, in attempting to correct a predicted printing problem, the invention goes beyond the prior art in that it can effect a change not only in the color of an edge but in its attributes such as rendering intent as well. This is important because an edge with two different rendering intents (resulting, for example, in different halftones being applied) can create printing problems even if the edge colors will not cause misregistration problems.
Fourth, in attempting to correct a predicted printing problem, the invention goes beyond the prior art in that it does not simply add a uniform-width, constant-color frame at an edge. Instead, it applies a function to the region near an edge which can change any of its attributes (color, rendering intent, etc.) as a function of the distance from the edge. This is important because it allows the system to solve more effectively a wider range of printing problems. For example, a xerographic printing problem called trail-edge deletion can occur when a color fill with sufficient size in the process direction is printed. At the lower process-direction edge, the toner is often depleted, resulting in a lighter color at the edge. This printing problem cannot be solved with a prior art frame or trap. However, by applying a function which changes the density of the color near the edge as a function of the scanline distance from the edge, the problem may be successfully pre-compensated for by the current invention. Note that the function is anamorphic: only process direction edges are modified. Even with the problem of misregistration, applying a function anamorphically to an edge can allow a subtler solution which might be seen as having the effect of a non-constant color, variable width frame different for each process or scan direction run of color, something not suggested by the prior art.
One object of the present invention is to provide a method for predicting when a printing problem is likely to occur based on adjacent colors and/or object attributes such as rendering intent, size, or extent in scan or process directions.
Another object of the present invention is to provide a method for correcting/pre-compensating for a predicted printing problem which can apply a different solution in the process direction (down the page) or the scan direction (across the page), in order to better address the majority of printing problems which are asymmetric (different in the scan and process directions).
Another object of the present invention is to provide a method for correcting/pre-compensating for a predicted printing problem which can effect a change not only in the color near an edge but in its attributes such as rendering intent as well. This allows an edge with two different rendering intents (resulting, for example, in different halftones being applied) to be detected and corrected if necessary even if the edge colors are deemed to be compatible.
Another object of the present invention is to provide a method for correcting/pre-compensating for a predicted printing problem which can apply a function to the region near an edge to change its attributes (color, rendering intent, etc.) as a function of the distance from the edge. This allows a wider range of printing problems to be corrected that cannot be solved with a prior art frame or trap.
To achieve the foregoing and other objects and to overcome the shortcomings discussed above, a digital color printing method and system is provided which analyzes adjacent runs of pixels to identify potential printing problems, using information of the adjacent runs of pixels such as the colors, rendering intents, size, and object type. Having predicted a set of potential printing problems, a method is provided to pre-compensate for the problems by applying functions to the identified problem runs of pixels which are anamorphic with respect to direction on the page, and which can modify various attributes of the pixels (color, rendering intent, object type, etc) as a function of distance of each pixel from the identified edge.
By applying such a function to the pixels near an edge, a zone can be created around an edge which effectively blocks or masks a variety of potential printing problems to increase overall image quality.